For example, this kind of battery-operated equipment is generally equipped with a micon in order to realize multifunction, when incorporated in an electrical shaver, a portable telephone or the like.
An electronic device described as a prior art in Japanese Laid-open Patent Publication No. H07-325633 is constructed with a battery, a regulator (3-terminal regulator) connected with the battery, and a main circuit that includes a micon (CPU) and is connected with the output of the regulator. In the electronic device, the regulator derives a constant voltage from a battery voltage across the battery to supply the main circuit with the constant voltage as an operating power source. Generally, the constant voltage as an output voltage of the regulator is lower than the battery voltage as its input voltage due to operating efficiency (loss on regulating) of the regulator. On account of this, when the battery voltage falls and the difference between the battery voltage and the constant voltage becomes small, there are cases where, even if the battery voltage is higher than a minimum operation voltage of the main circuit, the constant voltage becomes lower than the minimum operation voltage. In such cases, since the main circuit cannot operate in spite of the situation that the battery can, by itself, supply the battery voltage higher than the minimum operation voltage, it is necessary to exchange the battery for a new one or to recharge the battery.
A battery-operated equipment described in Japanese Laid-open Patent Publication No. H07-325633 includes a battery, a regulator and a main circuit as well as the above-described electronic device, and further includes a detection circuit and a changeover circuit. The detection circuit detects an output voltage of the regulator. The changeover circuit connects an output of the battery or an output of the regulator to the main circuit according to a control signal from the main circuit. The main circuit is provided with a control circuit (micon) that issues the control signal to the changeover circuit based on the output voltage detected by the detection circuit. Namely, when a value of the output voltage is within a specified value range of the main circuit, the control circuit issues a LOW signal (as the control signal) for switching a power source for the main circuit to the output of the regulator. Inversely, when a value of the output voltage is lower than the specified value range, the control circuit issues a HIGH signal (as the control signal) for switching the power source for the main circuit to the output of the battery. According to this configuration, since the power source for the main circuit is switched to the output of the battery if a value of the output voltage detected by the detection circuit is lower than the specified value range, it is possible to extend usable time of the equipment without exchange of the battery or recharge of the battery in comparison with the way that only output of the regulator is used as the power source for the main circuit.
However, since the above conventional battery-operated equipment requires the changeover circuit, its circuit configuration becomes complicated. Also, since the battery-operated equipment stops the operation of the regulator after the power source for the main circuit is switched to the output of the battery, it is unable to use an A/D converter(s) incorporated in the micon. This is because a reference voltage used by the A/D converter cannot be derived from the regulator.